ANATOMY AND AFFINITY OF CERTAIN RARE AND PRIMITIVE FERNS. 383 
stele proper and the decurrent bases of the leaf-trace strands. The perforations may 
run outwards for long distances into the leaf-trace bundles, as in the right of the 
reconstruction. When this occurs the abaxial margin of the leaf-trace strand is 
reduced to a slender strap decurrent from the leaf-trace strand to the stele. The 
ontogenetic development of these perforations has been followed. They arise by a 
local constriction of the endodermis, accompanied by parenchymatous replacement 
of both phloem and xylem in the constricted area. In the robust portions of the axis 
they frequently materialise, the endodermis being nipped across as the parenchy- 
matous gap in the phloem and xylem widens (text-fig. 24), but in the base of the 
leaf-trace and in the slender parts of the stele they commonly remain incipient 
(fig. 56 and text-fig. 23). It frequently happens, however, that a gap initiated in 
the stele may not close above in the leaf-trace. In these circumstances the slender 
marginal strand gradually diminishes, and finally ends blindly in the ground tissue of 
either axis or leaf-base (fig. 56). From the foregoing it will be seen that in- Gymno- 
gramme j aponica perforation of the stele is a progressive feature which may lead to 
further division of both stele and leaf-trace with the establishment of blindly-ending 
strands. This may have a direct bearing on the origin and interpretation of so-called 
“ compensation -strands ” commonly developed in certain types of divided leaf- 
traces. These “compensation-strands” may be in some cases the results of dissec- 
tion of the vascular system rather than provisions against mechanical or conductive 
difficulties. 
In the base of a strong petiole the leaf-trace is progressively reduced as regards 
actual and potential perforations. Within 3 inches of the leaf-base the changes 
